(-secretase is altered. Mutations in the presenilin genes PS1 and PS2 cause the most common form of early-onset familial Alzheimer's disease. The influence of PS1 mutations on the generation of endogenous intracellular amyloid -protein (A) species was assessed using a highly sensitive immunoblotting technique with inducible mouse neuroblastoma (Neuro 2a) cell lines expressing the human wild-type (wt) or mutated PS1 (M146L or exon 10). The induction of mutated PS1 increased the intracellular levels of two distinct A species ending at residue 42 that were likely to be A1-42 and its N-terminally truncated variant(s) Ax-42. The induction of mutated PS1 resulted in a higher level of intracellular A1-42 than of intracellular Ax-42, whereas extracellular levels of A1-42 and Ax-42 were increased proportionally. In addition, the intracellular generation of these A42 species in wt and mutated PS1-induced cells was completely blocked by brefeldin A, whereas it exhibited diffe rential se nsitivities to monensin: the increased accumulation of intracellular Ax-42 versus inhibition of intracellular A1-42 generation. These data strongly suggest that Ax-42 is generated in a proximal Golgi, whereas A1-42 is generated in a distal Golgi and/or a post-Golgi compartment. Thus, it appears that PS1 mutations enhance the degree of 42-specific -secretase cleavage that occurs in the normal -amyloid precursor protein processing pathway (a) in the endoplasmic reticulum or the early Golgi apparatus prior to -secretase cleavage or (b) in the distinct sites where Ax-42 and A1-42 are generated. Shinji Sudoh, Yuuki Kawamura, Shinji Sato, Rong Wang, Takaomi C. Saido, Fumitaka Oyama, Yoshiyuki Sakaki, Hiroto Komano, and Katsuhiko Yanagisawa (1998), J. Neurochem. 71, 1535-1543.